Heretofore, subsea guidance systems for interfacing a Christmas tree to a subsea wellhead have oftentimes included an upwardly facing funnel on the wellhead to guide and receive the lower end portion of the Christmas tree as the Christmas tree is lowered into a vertically aligned position with the subsea wellhead. The subsea wellhead normally includes a high pressure wellhead housing projecting upwardly from a low pressure wellhead housing or bowl.
The conventional vertical Christmas tree has a sturdy or robust connector at its bottom end for connection to a wellhead and can easily withstand vertical impacts associated with the installation of the tree on the wellhead assembly. Such impacts are likely to occur multiple times, because the surface vessel is subject to heaving in response to sea conditions. The tree is also likely to swing laterally in response to lateral movements during the positioning of the tree above the wellhead.
However, a horizontal Christmas tree often has a lower wellhead connector which includes an orientation sleeve projecting downwardly from the tree body for connection to a high pressure wellhead housing extending upwardly from the outer low pressure wellhead housing. The orientation sleeve must be aligned with the projecting high pressure wellhead housing and may be easily damaged if not properly aligned. The projecting orientation sleeve and particularly the lower sealing surface thereof is fragile and vulnerable to damage if not properly aligned, particularly upon improper contact with the projecting low pressure wellhead housing.
The most common technique for protecting the downward projecting orientation sleeve has been to provide a downwardly opening funnel on the tree body around the orientation sleeve for guiding the tree and the orientation sleeve into correct alignment with the wellhead. However, it has been found that damage to the orientation sleeve may occur even if a downwardly opening funnel is provided on the tree about the orientation sleeve, because geometric considerations associated with the greater wellhead system configuration may prevent optimum design for the guidance system. The orientation sleeve when installed typically extends downwardly between the tree body and the production casing hanger which is positioned inside the projecting high pressure wellhead housing and forms a fluid pressure barrier against the production casing hanger. The orientation sleeve is relatively long and may project a distance of about two (2) feet or more below the tree body. The orientation sleeve includes seals and is easily damaged, particularly if lowered onto an upwardly opening funnel on the wellhead.
It is desirable that a system for interfacing a tree and subsea wellhead be provided which does not require utilization of funnels on the wellhead or the tree.